ikev1: Don't queue more than one mode config or XAuth task

At the time we reset an IKE_SA (e.g. when re-authenticating a not yet
established SA due to a roaming event) such tasks might already be queued
by one of the phase 1 tasks.  If the SA is initiated again another task will
get queued by the phase 1 task.  This results in e.g. multiple mode config
requests, which most gateways will have problems with.
ikev1: Fix handling of UNITY_LOAD_BALANCE

The re-authentication is now handled within the original IKE_SA if it has not
yet been established, so we don't want to destroy it.
ipsec: Only set PATH if it is not already set

The comment indicated this but it was always set anyway.  All internal
commands are called via their absolute paths, so the script only uses PATH for
the uname command, but if that is not located in one of the configured
directories the script will fail.

Also, since the internal commands are called via their absolute paths there is
no need to add the directories to PATH.
ipsec: Remove duplicate check for internal commands
ipsec: Update usage output
ipsec: Document missing commands
ipsec: Remove unsupported listcards and rereadgroups commands